Alternate fuels have been developed to mitigate the rising prices of conventional fuels and for reducing exhaust emissions. For example, alcohol and alcohol-containing fuel blends have been recognized as attractive alternative fuels, in particular for automotive applications. Various engine systems may be used with alcohol fuels, utilizing various engine technologies and injection technologies. Further, various approaches may be used to control such alcohol-fuelled engines to take advantage of the charge-cooling effect of the high octane alcohol fuel, in particular to address engine knocking. For example, engine control methods may include adjustment of boost or spark timing in dependence upon the alcohol fuel, and various other engine operating conditions.
One example approach for controlling an alcohol-fuelled engine is illustrated by Oda et al. in U.S. Pat. No. 6,951,202. Therein, in response to engine knocking, an engine controller determines whether to inject a high octane fuel (e.g., ethanol fuel) into an engine cylinder to address the knock based on an amount of the high octane fuel in the fuel tank. Specifically, when a larger amount of high octane fuel is available, fuel injection is used instead of spark retard to address the knock. In comparison, when a smaller amount of fuel is available, spark retard is used instead of ethanol injection to address the knock.
However, the inventors herein have recognized a potential issue with such an approach. The biasing of engine knock control towards ethanol injection when a larger amount of ethanol fuel is available may lead to degraded volumetric fuel economy (miles per gallon) and sub-optimal engine performance. For example, under some conditions, even though sufficient ethanol fuel is available, using spark retard to address engine knock may provide better fuel economy. Similarly, there may be other engine knock conditions where even though sufficient ethanol fuel is available, it may be more advantageous to defer an ethanol injection until after a predefined amount of spark retard to provide an alternate benefit, such as lower exhaust emissions or lower price of engine operation (that is, more miles per dollar spent).
Thus in one example, the above issues may be addressed by a method of operating an engine comprising, in response to engine knock, retarding ignition spark timing up to a predetermined amount of retard, and increasing an amount of knock control fluid directly injected to suppress said engine knock after said ignition spark retard reaches the predetermined amount, while maintaining the spark retard at the predetermined amount. The knock control fluid may include one or more of gasoline, ethanol, methanol, other alcohols, water, washer fluid, other inert fluids, and combinations thereof.
In one example, a vehicle engine may be configured to use a plurality of fuels and fluid blends. For example, the engine may include a first injector for directly injecting a higher octane fuel (such as ethanol fuel blend E85) and a second injector for port injecting a lower octane fuel (such as gasoline). Based on a driver-selected cost function, such as higher fuel economy, lower exhaust CO2 emissions, higher miles per dollar spent, etc., an engine controller may adjust the usage of the ethanol fuel and gasoline so that the benefits of the ethanol fuel may be attained while using the fuels judiciously.
For example, to provide improved fuel economy benefits, in response to a knocking condition, the engine controller may address the knocking with at least some spark retard before direct injecting the higher octane fluid, herein the ethanol fuel. The controller may compare fuel economy losses due to spark retard with fuel economy losses due to the ethanol fuel injection to determine a threshold point, (herein also referred to as a break-even point or switch point), below which it may be more fuel efficient to retard spark when addressing knock, and above which it may be more fuel efficient to inject the ethanol fuel to address knock. The threshold point may be determined based on engine operating conditions (such as, an engine speed and load condition), as well as the alcohol content and/or the effective octane number of the fluid to be injected. As such, the threshold point may vary based on whether the threshold point was optimized for fuel economy, or an alternate cost function, such as reduced CO2 emissions, or reduced price of engine operation.
In one example, where the directed injected fluid has a higher effective octane content, such as the ethanol fuel E85, the break-even point may be determined to be 11 degrees of spark retard. Thus, in response to knocking, an engine controller may first address the knock by retarding spark up to 11 degrees of spark retard. Thereafter, the ignition timing may be held at 11 degrees of spark retard, and further knocking may be addressed by increasing direct injection of the ethanol fuel. In another example, where the direct injected fuel has a lower effective octane content, such as the ethanol fuel E10, the break-even point may be shifted to an earlier timing, such as 8 degrees of spark retard. Thus, in response to knocking, an engine controller may first address the knock by retarding spark up to 8 degrees of spark retard. Thereafter, the ignition timing may be held at 8 degrees of spark retard, and further knocking may be addressed by increasing direct injection of the ethanol fuel.
An amount of fluid injected to address the engine knock, after spark has been retarded to the predetermined timing, may be based on engine operating conditions as well as an octane required to address the knock and the effective octane content of the direct injected and port injected fluids in the engine's fuel system. The effective octane content of a fluid may represent the knock-addressing ability of the injected fluid taking into consideration the inherent octane content of the fuel, as well as other knock-mitigating characteristics, such as the fluid's effect on cylinder charge cooling and engine dilution. For example, as the effective octane content of a direct injected fuel increases, a smaller amount of direct injected fuel may be needed to address the knock.
In one example, the effective octane content of an injected fluid may be determined as a combination of a plurality of various knock-mitigating characteristics of the fuel, including, for example, an inherent fuel octane component representing the fuel's inherent octane number, an evaporative octane component representing the charge cooling effect of the fuel, and a dilution octane component representing the dilution effect of the fuel. Thus, when addressing knock, an effective octane content of each fluid in the engine's fuel system may be determined, and a threshold point between the use of spark retard and the use of a high octane fuel may be adjusted accordingly.
Additionally, or optionally, the effective octane number of an injected fluid may be determined based on the molar composition of the fuel. For example, where the injected fluid is a blend comprising at least two constituent fluids, the effective octane content (or an octane number) of the blend may be determined based on the volumetric fraction of each constituent in the blend, as well as the average molecular weight and density of each constituent.
In this way, based on a selected cost function, a controller may determine a corresponding threshold (or break-even point) above which injecting a higher octane content fluid may be more advantageous and below which an alternate engine adjustment (such as spark timing, valve timing, boost, etc.) may be more advantageous. By adjusting the threshold point based on all the knock-mitigating characteristics of the available fluids, engine performance and fluid usage may be improved, while providing all the knock-addressing benefits of the available fluid. Further still, by improving usage of a high octane fluid, the frequency of refueling of such a fluid may be reduced.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.